Reorganize varinfo files

[mohamed82008]

This PR doesn't introduce functional changes, it just moves some code around and puts them in files. This was part of #115.

[codecov]

Codecov Report

Merging #116 into master will not change coverage.
The diff coverage is 83.38%.

@@           Coverage Diff           @@
##           master     #116   +/-   ##
=======================================
  Coverage   74.83%   74.83%           
=======================================
  Files          14       17    +3     
  Lines         906      906           
=======================================
  Hits          678      678           
  Misses        228      228           

Impacted Files	Coverage Δ
src/DynamicPPL.jl	100.00% <ø> (ø)
src/varinfo/types.jl	70.58% <70.58%> (ø)
src/varinfo/utils.jl	82.94% <82.94%> (ø)
src/varinfo/linking.jl	88.46% <88.46%> (ø)
src/varinfo/indexing.jl	92.10% <92.10%> (ø)

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[yebai]

Changes in this PR are reverted. We discussed how to organise VarInfo a while ago, I still think a single file-based VarInfo with sections is easier to review and maintain. We can revisit this consensus in the future if necessary, but better to have some discussions before changing it.

[mohamed82008]

Sure we can revert this if you are really against it. But I am finding it difficult working on the varinfo.jl file as it is right now. It's a gigantic file that does a lot of things which can be grouped together nicely in files. I don't see why that's a bad thing.